1. Field of the Invention
The invention relates to methods and compositions for expressing and localizing anthranilate synthase in plant cells.
2. Description of Related Art
In maize, anthranilate synthase exists as a two-subunit enzyme which catalyzes the first reaction branching from the aromatic amino acid pathway to the biosynthesis of tryptophan in the chloroplast. It has been shown to be an important enzyme in the regulation of tryptophan production in plants. Anderson et al. (U.S. Pat. No. 6,118,047) demonstrated that over expression of a tryptophan-insensitive α-subunit of anthranilate synthase from maize led to an increased level of tryptophan in transgenic maize plants. Recently, it has been shown that monomeric forms of anthranilate synthases from prokaryotic sources are capable of increasing tryptophan levels in transgenic soybeans and corn (U.S. patent application Ser. No. 10/138,927, issued as U.S. Pat. No. 7,217,865, and Ser. No. 10/430,011, published as US Patent Application Publication 20030213010).
Most proteins which participate in the biosynthetic pathways within the chloroplast are nuclear-encoded and are synthesized in the cytosol. Correct targeting of these proteins to the plastids is thus essential for their biosynthetic function. In most cases, this targeting is achieved by the presence of an N-terminal extension, called a chloroplast transit peptide (CTP) or plastid transit peptide. Transgenes from bacterial sources must have a sequence encoding a CTP sequence fused to a sequence encoding an expressed polypeptide if the expressed polypeptide is to be compartmentalized in the chloroplast. Accordingly, transport of an exogenous polypeptide to a chloroplast is accomplished by means of operably linking a polynucleotide sequence encoding a CTP sequence to the 5′ region of a polynucleotide encoding the exogenous polypeptide.
For many purposes in the manipulation and transformation of plant cells with a monomeric anthranilate synthase, it will be desirable that the gene that is introduced into the plant cell results in a product that is translocated to the plastid and functions in the plastid. Not all CTPs, however, are able to accomplish this translocation with equal efficacy. The identification of efficient and effective CTPs for successful expression and localization of anthranilate synthase in monocotyledonous plants, and in particular maize plants, is needed in the art.